Method and apparatus for electroless plating a contact pad

ABSTRACT

A method and apparatus is disclosed for sequential processing of integrated circuits, particularly for conductively passivating a contact pad with a material which resists formation of resistive oxides. In particular, a tank is divided into three compartments, each holding a different solution: a lower compartment and two upper compartments divided by a barrier, which extends across and partway down the tank. The solutions have different densities and therefore separate into different layers. In the illustrated embodiment, integrated circuits with patterned contact pads are passed through one of the upper compartments, in which oxide is removed from the contact pads. Continuing downward into the lower compartment and laterally beneath the barrier, a protective layer is selectively formed on the insulating layer surrounding the contact pads. As the integrated circuits are moved upwardly into the second upper compartment, a conducting monomer selectively forms on the contact pads prior to any exposure to air. The integrated circuits can then be transferred to an ozone chamber where polymerization results in a conductive passivation layer on the contact pad.

REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. application Ser. No.09/256,548, filed Feb. 24, 1999.

FIELD OF THE INVENTION

The invention relates generally to contact pads in integrated circuits,and more particularly to oxide-free bond pads.

BACKGROUND OF THE INVENTION

Bond pads are electrical terminals which connect an integrated circuitdie or chip to the electrical system outside of the chip. The electricalconnection is normally made by bonding electrical leads to the bond pad.The chip can then be connected to a larger circuit, such as a printedcircuit board (PCB), with the leads making contact with the outsidesystem.

The bond pads are integrally connected to metal lines or runners withinthe die, which are typically formed of a metal such as aluminum,aluminum-silicon eutectic, aluminum-copper alloys, or polysilicon. Thebond pads themselves are also typically formed of aluminum or analuminum alloy, which is highly conductive and relatively inexpensive.Unfortunately, aluminum or aluminum alloy readily oxidizes to formaluminum oxide. The aluminum oxide is not conductive, and it thereforeincreases the overall resistivity of the system. Increased resistivity,in turn, leads to slower signal propagation.

Conventionally, aluminum oxide is removed with a reducing agent inseveral separate steps. The chip is exposed to atmosphere between steps,and the exposed metal spontaneously oxidizes, impairing the conductiveconnection. Even the short exposure between oxide cleaning and sealingthe bond pad results in aluminum oxide formation between the metal andsealant.

There is thus a need for a method of avoiding oxide on the surface of acontact pad.

SUMMARY OF THE INVENTION

In view of this need, the present invention provides a method andapparatus for providing conductive passivation on contact pads, such asbond pads.

In accordance with one aspect of the invention, a method is provided forplating a conductive layer in an integrated circuit. The method includesimmersing the integrated circuit in a cleaning fluid. The integratedcircuit is then transferred from the cleaning fluid to a plating fluid,without exposing the integrated circuit to air.

In an illustrative embodiment, such transfer is performed directly fromone liquid phase to another. The cleaning fluid represents a firstliquid phase, preferably an oxide etch bath, and the second liquid phaseforms a protective layer over the insulating material which surroundsthe contact pad. The plating fluid is in yet a third liquid phase,containing a conducting monomer in solution. This forms a monomer layerover the conductive layer, which is later polymerized to form aconductive polymer. The integrated circuit sequentially moves betweenthe first and second phases, and between the second and third phases,without passing through air. As will be understood by the skilledartisan, such an arrangement enables sealing the underlying conductivelayer of the contact pad, which may be susceptible to oxidation,immediately after oxide removal. Nether oxide nor other contaminantshave the opportunity to form on the conductive layer between steps,which would hinder electrical contact between the contact pad andoutside circuits.

In accordance with another aspect of the invention, an apparatus isprovided for sequential processing with two or more liquid solutions.The apparatus includes a water-tight tank with an upper portion and alower portion. The upper portion is divided into at least a first sideand a second side by a water-tight barrier. The lower portion is open toand extends beneath both the first side and the second side.

This apparatus is particularly useful for the illustrated process, whereone side of the upper portion holds an oxide cleaning agent (e.g., 1%NaOH, density about 1.0 g/cm³) and the other side of the upper portionholds a conducting monomer in solution (e.g., pyrrole, density less thanabout 0.99 g/cm³). The barrier separates the cleaning solution from themonomer solution. The lower portion holds a relatively more densesolution for forming a protective layer (e.g., siliconizing solution,density about 1.09 g/cm³), ensuring that the phases are naturallyseparated by gravity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of partially fabricatedintegrated circuit, showing a bond pad covered with an oxide layer.

FIG. 2 illustrates the integrated circuit of FIG. 1 after the oxidelayer has been removed, exposing the conductive layer.

FIG. 3 illustrates the integrated circuit of FIG. 2 after a protectivelayer has been formed on the dielectric layer.

FIG. 4 illustrates the integrated circuit of FIG. 3 after formation of apassivation precursor on the conductive layer.

FIG. 5 illustrates the integrated circuit of FIG. 4 after treatment ofthe precursor layer, forming a conductive passivation layer on theconductive layer.

FIG. 6 is a process flow diagram showing the process steps and movementof the integrated circuit while forming the conductive passivation onthe bond pad.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present application describes a method of cleaning and passivating aconductive surface with a conductive layer without exposing the surfaceto air between the process steps. While illustrated in the context ofconductive passivation of aluminum bond pads, the skilled artisan willrecognize many other applications for the methods and structuresdisclosed herein. In particular, contact pads formed of other metals,while less susceptible to oxidation, will also benefit from theconductive passivation of the illustrated embodiment. Furthermore,sequential cleaning and conductive passivation, without allowingre-oxidation, will have utility for a great many applications beyondintegrated bond pads.

Initially, an integrated circuit is formed on a substrate. The substrateincludes a semiconductor layer or wafer, such as silicon or galliumarsenide, in which active or operable portions of electrical devices areformed. Through a series of mask, etch and deposition steps, electricaldevices such as transistors, capacitors and resistors are integrallyformed and interconnected by metal layers separated by insulatinglayers. Typically, a plurality of chips or dies are formed in a singlewafer.

Upper levels of the metal interconnections are terminated in integralbond pads for forming connections between the chips or dies and outsidecircuits. The bond pads comprise conductive layers surrounded byinsulating layers. Similar contact pads, typically referred to as “probepads,” are often formed in lower layers for testing circuits atintermediate steps in fabrication.

The conductive layers can be metal, silicide, or other suitableconductive material. Some examples of conductive layers include, but arenot limited to, copper, gold, aluminum, doped silicon and the like.Mixtures of metals are also suitable for forming a conducting layer.Some suitable mixtures of metals include, but are not limited to,aluminum alloys formed with copper and/or silicon.

FIG. 1 illustrates such a conductive layer 10 formed and patterned intoa bond pad over metal layers and devices formed in a substrate (notshown). A window 15 is formed in a surrounding insulating layer 20 toexpose the conductive layer 10. The illustrated insulating layer 20 is adielectric material suitable for final passivation, such as silicondioxide, silicon nitride or silicon oxynitride. In the illustratedembodiment, the insulating layer comprises silicon nitride (Si₃N₄),which is known to have good moisture barrier qualities.

The conductive layer 10 of the illustrated embodiment comprisesaluminum, and particularly aluminum mixed with copper. In a preferredembodiment, the conductive layer comprises aluminum with about 0.5%copper content. The illustrated conductive layer 10 is particularlysusceptible to oxidation. An oxide layer 30 thus naturally forms on thesurface of the conductive layer 10, as shown in FIG. 1, upon exposure toair, such as after deposition of the conductive layer 10. Theillustrated oxide layer 30 comprises aluminum oxide, which prevents orhinders electrical conduction front the conductive layer 10 to contactsformed thereupon, such as wire bonds or solder balls. It is thereforepreferred to remove the oxide layer 10 before attaching a conductivecontact (e.g., pins, wires, solder balls, etc.) to the bond pad.

Traditionally, removal of the oxide layer 30 and formation of a contactcomprise several process steps, and even brief exposure to air betweensteps results in re-oxidation and/or other contamination of theconductive layer 10. For the illustrated conductive layer 10, oxidationof the aluminum in the presence of air is almost instantaneous. Theembodiments of the present invention provide a method of removing theoxide layer 30 and passivating the exposed conductive layer 10 with aconductive polymer in situ, without exposing the conductive layer 10 toair or other contaminants.

The oxide layer 30 can be removed in a variety of ways. Typically, theoxide layer 30 is exposed to a reducing agent. In the illustratedembodiment, the substrate is immersed in a dilute base solution (e.g.,between about 8 pH and 14 pH). An exemplary solution for removing theoxide layer comprises approximately 1% NaOH in water. The bath ispreferably between about 20° C. and 50° C., more preferably betweenabout 20° C. and 30° C. The wafer is preferably immersed in the solutionfor between about 0.2 and 30 minutes, more preferably between about 1and 10 minutes.

FIG. 2 shows the integrated circuit after removal of the metal oxide 30from the top of the conductive layer 10. Removal of the metal oxide 30exposes the conductive layer 10, as shown. If exposed to air, the metalin the conductive layer 10 would spontaneously re-oxidize, forming a newmetal oxide layer. In accordance with the preferred method, however, theconductive layer 10 is not exposed to air or other contaminants afterremoval of the oxide 30, as will be apparent from FIG. 6 and theaccompanying text.

With reference to FIG. 3, a protective layer 40 is then formed on theinsulating layer 20. The protective layer 40 is selectively formed onthe insulating layer 20 without forming on the exposed conductive layer10. In the illustrated embodiment, such selectivity is accomplished byimmersing the integrated circuit in a siliconizing agent. Thesiliconizing agent can comprise a wide variety of compounds. Anexemplary siliconizing agent is dichloro-octamethyl-tetrasiloxane,commercially available from SurfaSil™ of Rockville, Ill.

Chlorine ions in the preferred siliconizing agent are attracted tosilanol groups on the surface of the preferred silicon nitrideinsulating layer 20, essentially forming a monolayer of the siliconizingagent. The siliconizing agent, however, does not bond to the metal inthe conductive layer 10. The siliconizing agent also has the advantageof continuing to clean the surface of the conductive layer 10. Moreover,exposed methyl tails of the illustrated protective layer 10 arehydrophobic, which facilitates later selective formation of theconductive passivation, as will be understood better from the discussionbelow.

With reference to FIG. 4, a passivation precursor layer 50 is thendeposited onto the exposed surface of the conductive layer 10. Theillustrated precursor layer 50 comprises a conducting monomer, andparticularly pyrrole (C₄H₅N), though other monomers such as acetylene oraniline can also be used. The pyrrole does not deposit onto the surfaceof the dielectric layer 20 due to the intervening protective layer 40.In particular, the hydrophobic upper surface of the protective layer 40prevents pyrrole from depositing on the nitride 20, while hydrophilicinteractions cause deposition on the conductive layer 10.

With reference to FIG. 5, the precursor layer 50 is then treated toresult in a conductive passivation layer 60 directly on the surface ofthe conductive layer 10, with no intervening oxide. In the illustratedembodiment, such treatment comprises polymerizing the monomer of thepreferred precursor layer 50, leaving a conductive polymer in its place.As will be readily appreciated by the skilled artisan, polymerization ofthe preferred precursor can be accomplished by exposure to an oxidationagent, such as ozone or permanganate.

As also shown in FIG. 5, the protective layer 40 can be removed at thispoint. The illustrated protective layer 40 can be removed by applicationof heat, which evaporates the monolayer on the surface of the insulatinglayer 20. The flash point for evaporation ofdichloro-octamethyl-tetrasiloxane is about 78° F.

The resulting polymer 60 is non-oxidizing and therefore passivates thesurface of the conductive layer 10, completing the electroless bond padplating. At the same time, the conductive polymer 60 serves to provide aconductive surface to which wires, pins or solder balls can be attachedprior to die encapsulation. The bond pad can be thereby electricallyconnected to outside circuitry, such as the motherboard of a personalcomputer.

FIG. 6 schematically illustrates a preferred process and apparatus forforming conductive passivation for integrated contact pads. Forsimplicity, reference numeral 70 will be utilized to refer to anintegrated circuit in which the contact pad is integrally formed. Thedescribed process begins after the integrated circuit 70 has beenfabricated to the point of having windows opened in an insulating layerto expose an underlying conductive layer. The conductive layer haspreferably already been patterned into a contact pad, as shown anddiscussed with respect to FIG. 1, though such patterning may also beconducted after forming the conductive passivation of the presentinvention. As also discussed above, the illustrated conductive layer 10includes metal, particularly aluminum, and has a resistive oxide 30formed thereover.

It will be understood that the integrated circuit 70 may be anindividual die, or it may represent a wafer with a plurality of diesprior to separation. In either case, the process is most efficientlyperformed simultaneously on a plurality of dies or wafers in a boat orother carrier.

Both the process flow and the physical movement of the integratedcircuit are illustrated in FIG. 6 by a path 72. The integrated circuit70 is lowered into a water-tight tank or container 75, which holds aplurality of treatment phases. The container 75 is preferably formed ofa robust material which can withstand the chemicals in each of thetreatment phases, and is preferably made of or lined with Teflon™.Movement through the phases is preferably accomplished by known roboticmechanisms.

A water-tight barrier 80 divides the container 75 into at least twocompartments, and preferably greater than two, each holding a differenttreatment phase. Preferably, the phases are liquid solutions which areimmiscible, differ in density, and are arranged for sequentialprocessing of the integrated circuit 70. Immiscible solutions, as thatterm is employed herein, refers to solutions which do not dissolve inone another. Aqueous solutions, for example, tend to be immiscible withsolutions having organic solvents, although many other solutions arealso immiscible. It will be apparent to the skilled artisan, in view ofthe present disclosure, that immiscible gaseous phases of differingdensities, devoid of intermediate contaminating media, can also bearranged.

In the illustrated embodiment, the container 75 and barrier 80 definethree compartments. A first compartment 85, in the upper portion of thecontainer on a first side of the barrier 80, holds a cleaning agent forcleaning the surface of the contact pad. A second compartment 90, belowthe first compartment 85 and extending below the barrier 80, holds asolution for selectively forming a protective layer over the insulatinglayer surrounding the contact pad. A third compartment 95, extendingabove the second compartment on a second side of the barrier 80, holds athird solution for selectively forming a precursor layer on the contactpad. Desirably, the third compartment leads to a chamber in which theprecursor layer can be treated to form the conductive passivation forthe contact pad.

It will be understood that, in other arrangements, the protective layermay be unnecessary. For example, some precursor materials or finalconductive passivation materials can be formed on the contact padselectively without forming on the surrounding insulating material.Furthermore, a precursor layer need not be employed where the conductivepassivation material can be formed directly, without curing orpolymerizing treatments.

Returning to the illustrated embodiment, the preferred cleaning agent inthe first compartment 85 comprises an oxide etchant to clean the metaloxide from the conductive layer and expose the underlying conductivelayer. In other arrangements, the cleaning agent may remove othercontaminants, such as carbon or sulfur. As discussed above, aluminumoxide is preferably removed by a reducing agent, and in the preferredembodiment the first compartment 85 holds a dilute base solution such as1% aqueous NaOH, which has a density of about 1.0 g/cm. The integratedcircuit 70 is lowered into first compartment 85, cleaning the oxide fromthe surface of the conductive layer. It will be understood that the term“lowered” is meant in encompass movement of the container 75 relative toa stationary integrated circuit 70.

The integrated circuit 70 is further lowered to the second compartment90, where the second solution selectively forms a protective layer onthe insulating material surrounding the contact pad. In the preferredembodiment, the surrounding insulating material comprises siliconnitride, and the second solution comprises a siliconizing agent. Thepreferred siliconizing agent forms the protective layer with ahydrophobic upper surface, while continuing to clean the metal of thecontact pad.

The preferred solution (dichloro-octamethyl-tetrasiloxane, commerciallyavailable under from SurfaSil™ of Rockville, Ill.) is an organicsolution having a density of about 1.09 g/cm³. The second solution doesnot mix with the overlying aqueous NaOH in the first compartment 85, andnaturally rests below the aqueous NaOH, and no physical barrier orintermediate chamber is required to separate the phases. The integratedcircuit 70 thus passes from a first phase in the first compartment 85 toa second phase in the second compartment 90, without exposure to air orother contaminants.

After moving laterally through the illustrated second compartment 90,the integrated circuit 70 is then moved upwardly into the thirdcompartment 95. In this phase, the preferred precursor layer is formedon the cleaned conductive layer of the contact pad. As described above,the solution in the third compartment 95 comprises a conducting monomerto be later treated to form a conductive polymer. The monomer depositson the conductive layer through hydrophilic interactions, while theprotective layer over the surrounding insulating material preventsmonomer deposition thereover.

The preferred monomer comprises pyrrole, having a density of less thanabout 0.99 g/cm³. Accordingly, the pyrrole preferably floats above thepreferred siliconizing agent in the underlying second compartment 90. Aswith the transfer from the first compartment 85 to the secondcompartment 90, the integrated circuit 70 need not pass throughintermediate contaminating media, such as air, prior to forming theprecursor layer.

Note that, in the illustrated embodiment, the phase in the thirdcompartment 95 need not be immiscible with or of a different densitythan the phase in the first compartment 85, since the barrier 80physically separates these phases. To ensure physical separation of thecleaning solution in the first compartment 85 from the monomer solutionin the third compartment 95, the siliconizing agent in the lowercompartment 90 preferably overfills the lower compartment 90 and extendsinto each of the upper compartments 85, 95, as illustrated.

As illustrated, the integrated circuit 70 is then raised out of thethird compartment 95 into a fourth phase, where the precursor layer istreated to form the conductive passivation. As discussed above, theillustrated treatment comprises polymerization of the conductingmonomer, and the preferred fourth phase comprises an ozone chamber.Desirably, heat treatment is also applied, which serves to evaporate theillustrated protective layer from over the surrounding insulatingmaterial.

Various modifications of the embodiment of FIG. 6 may be made withoutdeparting from the spirit of the invention. For example, the density ofthe phases can be reversed, such that the densities of the cleaningsolution and the conducting monomer can be greater than the density ofthe solution forming the protective layer.

It will be understood that the integrated circuit 70 can pass throughintermediate media prior to the polymerization or evaporation steps, ifdesired. Moreover, the integrated circuit may be passed throughintermediate phases between the first and second or second and thirdphases, though such intermediate phases are preferably devoid ofoxidizing or other contaminants. For example, an intermediate liquidphase can be added between the cleaning agent phase and the phase inwhich the protective layer is formed. Such an intermediate phase couldperform additional cleaning of the conductive layer surface withoutexposing the same to air or other contaminants.

Though described in terms of certain preferred embodiments, the skilledartisan will readily appreciate that various modifications andalterations may be made to the described processes and structures,without departing from the scope and spirit of the invention.Accordingly, the invention is not meant to be limited to the embodimentsdisclosed herein, but should rather be defined by reference to theappended claims.

We claim:
 1. A sequential, liquid phase treatment apparatus, comprising:a tank having a lower compartment, an upper first compartment, and anupper second compartment adjacent to and separated from the upper firstcompartment by a barrier, wherein the lower compartment extends beneathand is open to the upper first compartment and the upper secondcompartment; a cleaning solution held in the upper first compartment; asiliconizing solution held in the lower compartment; and a platingsolution held in the upper second compartment.
 2. The apparatus of claim1, wherein the cleaning solution comprises an oxide etchant.
 3. Theapparatus of claim 1, wherein the siliconizing solution comprisesdichloro-octamethyl-tetrasiloxane.
 4. The apparatus of claim 1, whereinthe plating solution comprises a conducting monomer, the apparatusfurther comprising an ozone chamber.
 5. The apparatus of claim 1,wherein the tank comprises sidewalls formed of polytetrafluoroethylene.6. An apparatus for sequential in situ cleaning and formation of aconductive passivation layer on a conductive element in an integratedcircuit, the apparatus comprising: a tank; a first liquid treatmentphase within the tank, comprising an aqueous cleaning solution having afirst density; a second liquid treatment phase within the tank, thesecond phase having a second density different from the first density,the second phase being immiscible and in direct contact with the firstphase, the second phase is more dense than and extending under the firstphase; and a third liquid treatment phase having a third densitydifferent from the second density, the third phase being immiscible withthe second phase and in direct contact therewith.
 7. The apparatus ofclaim 6, wherein the second phase selectively forms a protective layerover an insulating layer surrounding the conductive element and thethird phase selectively forms a conducting material over the conductiveelement.
 8. The apparatus of claim 7, wherein the conductive elementcomprises a bond pad for connecting the integrated circuit to an outsidecircuit.
 9. The apparatus of claim 6, wherein the second phase comprisesan organic solvent.